The simultaneous use of an analog display by means of time indicating hands or discs and of a digital display within a watch has rapidly expanded over recent years. This type of watch effectively provides the advantage of enabling reading the time in an easy and conventional manner with the time indicating hands and permitting access to numerous auxiliary functions such as the day, date, alarm time, time zones, ect. on the digital display.
For certain functions the information items of the two displays are different and independent as for instance the time read on the analog display and a chronograph or measured time indicated on the digital display. For other functions both displays may give the same information items which depend on one another. For example, the day and the data read on the digital display must be related to the time of the analog dispaly in order that the calendar may change its state when the hands indicate midnight. Among the various functions of the watch, one function must thus enable one to read and correct the time indicated by the hands independently from the time indicated on the digital display or inversely. Even if this information is not explicitly employed, it is necessary for the calendar function as well as, for example, for the time zone function.
When the same information item is indicated on both displays, the time for instance, there must thus exist display synchronization or phasing means, so as to enable to have appear in this type of watch the same time on the analog and digital display. This operation is necessary at the time when the watch is first put into service and each time when the battery is changed.
Two types of watches provided with analog and digital displays are for instance described in the U.S. Pat. No. 4,246,602. In the first version the analog display is corrected mechanically by means of a crown, independently from the digital display which continues to receive time signals. This manner of operation is at the least imprecise because of the play in the gearing, necessitates a clumsy and expensive mechanism and prevents use of the crown for functions other than that described. In the second version the analog display is stopped by means of a switch and it is the digital display which is corrected with the help of contacts until there is coincidence between both indications. The watch in this case does not have a crown. An exact synchronization may be obtained with an analog hour display, the minutes advancing by integral units. On the other hand, if the minutes hand advances by fractions of a minute as is most often the case, the exactness of the synchronization cannot be guaranteed and one arrives back at the first difficulty of the initial version.
The British laid open patent application GB No. 2 019 052, on the other hand, described a watch having a mixed analog-digital display provided with a crown enabling electronic correction backwards and forwards of both displays by integral minute steps. This is an interesting solution since it approaches the usual type of well accepted control employed in mechanical watches. The watch described presents however the disadvantage of requiring in addition to the crown further control mechanisms and to lack means which permit synchronizing the displays.